Here's why:

* High organic matter content: Mollisols are characterized by a thick, dark-colored topsoil rich in organic matter. This organic matter provides essential nutrients for plant growth and improves soil structure.

* Excellent water-holding capacity: The high organic matter content also enhances the soil's ability to hold water, making it ideal for plant growth.

* Nutrient-rich: Mollisols are naturally rich in nutrients like nitrogen, phosphorus, and potassium, which are essential for plant development.

* Found in temperate grasslands: These soils are typically found in regions with temperate climates and grasslands, where grasses decompose and contribute to the high organic matter content.

While Mollisols are considered highly fertile, it's important to remember that soil fertility can vary depending on factors like climate, drainage, and management practices.